Assessing the Long-Term Stability of a Dispersion or Emulsion

The evaluation of the long-term stability of an emulsion or dispersion can be a long and complicated process. Nevertheless, it is important to make sure that a product meets the required quality standards. This article describes the method and data used for assessing the stability of two shower gel products, versus their ability to suspend bubbles as a product requirement.

Stability is typically achieved using a combination of effects, including: increasing electrostatic or steric repulsion of the dispersed phase, decreasing interfacial tension, and/or increasing the continuous phase viscosity.

The collective effect of these factors in dilute dispersions is usually reflected in the zero shear viscosity, which in turn indicates the rate at which dispersions will settle or droplets will combine and divide.

In the case of concentrated systems, a network structure can be formed either through droplet/particle jamming or dispersed phase interaction. In such a situation, stability will mainly correspond with the strength of the network structure. This structure can then be determined by the yield stress.

In order to achieve stability, it is important that the yield stress is higher than the stress induced by the dispersed phase under the gravitational effect. This can be calculated from the equation given below:

         

Where pD is the density of dispersed phase; pC is the density of continuous phase; r is the radius of dispersed phase; and g is the acceleration due to gravity.

Several experimental tests can be used to establish the yield stress. Shear stress sweep is a fast and simple test which can be carried out to measure the stress at which a viscosity peak is seen.

The material normally goes through elastic deformation before this viscosity peak. Hence, this peak indicates the point at which the elastic structure collapses and the material starts to flow.

To achieve a stable system, the yield stress should be strong enough to tolerate the stresses caused by the dispersed particles; however, additional stresses may be possibly encountered during the course of product transportation.

Experimental Framework

In this analysis, two shower gel products (bodywash) that are commercially available in the market were assessed - one containing a surfactant and an associative thickener and the other containing only the surfactant.

The former product has been prepared so that it can suspend the bubbles in the bottle while the product remains on the shelf. This was done to prevent the effect of the bubbles on the rheological behaviour; before testing, the bubbles that were present in the sample were removed through centrifugation.

Next, the stress caused by a dispersed particle on the surrounding medium was measured by using a particle stress calculator in the rSpace software, with particle properties as user inputs (Equation 1).

A Kinexus rheometer equipped with a cone and plate measuring system and a Peltier plate cartridge was then used to perform rotational rheology measurements. Normal pre-configured sequences in the rSpace software were also used to make these measurements. Additionally, a standard loading sequence was utilized to make sure that both samples go through a controllable and reliable loading procedure.

All rheology measurements were carried out at 25°C temperature. Following this, a shear stress ramp was carried out and the data obtained was studied by means of a peak analysis to measure the yield stress.

Finally, the extent of the product yield stress is evaluated against the imposed stress, which was measured from the dispersed particulate properties to evaluate long-term stability of the system.

Results and Discussion

The viscosity versus stress curves for both shower gel samples in the stress ramp test is shown in Figure 1. Data obtained for Bodywash 1 shows a flat viscosity peak in the stress ramp test, whilst data obtained for Bodywash 2 displays a distinct viscosity peak.

This indicates that Bodywash 1 behaves similar to a liquid having a zero shear viscosity, while Bodywash 2 has strain hardening relating to a yield stress. Although viscoelastic liquids do not have an actual yield stress, they may still reveal a slight peak in viscosity. In such a situation, either user discretion may be needed or confirmation through a table of shear rates test or a creep test may be required to validate the presence of a zero shear viscosity. The quantified yield stress for Bodywash 2 was found to be 4Pa.

Viscosity against shear stress curves from a stress ramp test for shower gels with (Bodywash 2) and without (Bodywash 1) associative thickener - the viscosity peak shown by Bodywash 2 data is indicative of a yield stress of 4Pa.

Figure 1. Viscosity against shear stress curves from a stress ramp test for shower gels with (Bodywash 2) and without (Bodywash 1) associative thickener - the viscosity peak shown by Bodywash 2 data is indicative of a yield stress of 4Pa.

On the basis of Equation 1, it can be estimated that the stress caused by a 100pm diameter air bubble will be roughly 0.65Pa and therefore a yield stress of 4Pa will be adequate to suspend the bubble phase. However, possible reduction in the network strength owing to increased temperature plus extra stresses encountered during the course of transport also need to be taken into account.

Given that Bodywash 1 does not possess a yield stress, an exact value of zero shear viscosity is required to assess the stability of the system, for example, analysis from a creep test. The data obtained from this test displayed the zero shear viscosity to be 8Pas.

From this figure, a bubble rise rate of about 6cm per day was estimated for a 100pm diameter air bubble. This is not acceptable to sustain the long-term stability of the dispersed system. The integration of a yield stress would be mainly to give the preferred shelf-life and long-term stability for a product that suspends bubbles.

Conclusion

A yield stress ramp test was performed to compare two shower gel products. Bodywash 1 without the thickener possessed a zero shear viscosity that was not adequate to promote long-term stability, while Bodywash 2 containing an associative thickener had a yield stress that can suspend the gas bubbles.

Therefore, the test provides a fast, easy and accurate method to predict the stability of a suspension for a specified particle density and size.

This information has been sourced, reviewed and adapted from materials provided by Malvern Panalytical.

For more information on this source, please visit Malvern Panalytical.

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